The planet Mercury is regularly being showered by bits of an ancient comet, according to a new study published in the journal Geophysical Research Letters. The material was left by comet Encke when it crossed the orbit of the planet thousands of years ago.
Scientists have noticed how more calcium is released by Mercury at certain times of the year. The astronomers thought Encke (which orbits the Sun every 3.3 years) was responsible for it, but to prove it, they had to simulate how the dust was released throughout the comet's recent history. They discovered that old material from the comet is located in almost the right position around the orbit to explain the extra calcium observed.
As the comet's fragments seasonally rain on the planet surface, they vaporize rocks and release elements into Mercury’s exosphere. An exosphere is essentially a very thin atmosphere, an envelope of molecules surrounding a planetary body that are gravitationally bound to it, but they are so spread out that don’t act like a gas. Mercury is small, close to the Sun and very hot, so wasn't able to hold onto a thick atmosphere like ours,
In the last 30 years, scientists discovered that Mercury’s exosphere contains sodium, potassium, calcium, and magnesium, and proposed regular impacts as the mechanism to replenish the exosphere. Using the MESSENGER probe, scientists discovered that calcium seemed to be more abundant after Mercury’s closest approach to the Sun (perihelion), but they could not explain why.
Most of the impacts were thought to be produced by zodiacal dust, fine material produced by comets and asteroid collisions, orbiting in the inner Solar System, because comet Encke's current path crosses Mercury's orbit a week after the planet’s perihelion, suggesting it was not the cause.
But the researchers wondered if the stream of particles from the comet might have shifted over time, so they ran a complex simulation modelling how the comet’s orbit has changed in the last 10,000 years, and examined the subtle interaction between the cometary dust and sunlight. Using the model, the team showed that the calcium abundance peak matches with dust ejected by comet Encke between 10,000 and 20,000 years ago.
“We already knew that impacts were important in producing exospheres,” Dr Rosemary Killen, one of the co-authors of the study, said in a statement. “What we did not know was the relative importance of comet streams over zodiacal dust. Apparently, comet streams can have a huge, but periodic, effect.”
The team now want to explore if the comet dust affects other elements in Mercury’s exosphere.